1. Field of the Invention
An embodiment of the present invention relates to a semiconductor device including an oxide semiconductor and a method for manufacturing the semiconductor device.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (TFT) using a semiconductor thin film (having a thickness of about several tens of nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and thin film transistors that are used as switching elements in image display devices are particularly being developed urgently. There are various kinds of metal oxides, which are used for a wide range of applications.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Thin film transistors in each of which such a metal oxide having semiconductor characteristics is used for a channel formation region have already been known (Patent Documents 1 and 2).
As for an oxide semiconductor used in such a transistor, there is description as follows: an oxide semiconductor is insensitive to impurities; there is no problem when a considerable amount of metal impurities is contained in the film; and soda-lime glass which contains a large amount of alkali metal such as sodium and is inexpensive can also be used (see Non-Patent Document 1).